The present invention relates to a synchronization detection system and, more particularly, to a synchronization detection system used in a digital transmission system for trunk transmission, public network, or subscriber network.
Transmission techniques using optical fibers as transmission media have been developed and advanced, and transmission at several hundreds of Mbps to several Gbps has become possible. A time-divisional multiplex system can be used to effectively utilize a digital transmission system having a large capacity. However, since high-speed processing is required, frame formatting is simplified as much as possible to obtain a compact, simple circuit arrangement. A technique for simplifying frame formatting is a time-divisional bit multiplex system, a general configuration of which is illustrated in FIG. 1. Referring to FIG. 1, one frame consists of K bits, and each bit is assigned to one channel. One of the K channels is assigned to the frame channel. Referring to FIG. 1, reference symbol F denotes a frame channel. Bit positions #1 to #K-1 are respectively assigned to (K-1) channels. According to this system, a specific frame pattern is inserted in the frame channels F of a few frames in order to perform bit multiplexing. At the time of synchronization detection, data signals are separated in units of channels, and a signal train separated from any channel is checked to determine whether it coincides with the specific frame pattern inserted in the frame channel F, thereby detecting the frame channel and hence synchronization.
According to another conventional technique for simplifying frame formatting, a frame is divided into a plurality of sub-frames, and the frame pattern is distributed into the sub-frames. A frame employing this technique is shown in FIG. 2. Referring to FIG. 2, one frame is divided into L sub-frames, and each sub-frame comprises I bits, so that one frame consists of I.times.L bits. The bits constituting the frame pattern are sequentially inserted in most significant bits (MSBs) of the sub-frames. Reference symbol F.sub.i (i=1, 2, . . . L) denotes a frame bit inserted in the MSB of each sub-frame; and #1 to #L, sub-frames each consisting of I bits. According to this system, (F.sub.1 F.sub.2 F.sub.3, . . . F.sub.L-1 F.sub.L) is the frame pattern. In order to detect synchronization or an alignment signal, the frame pattern (F.sub.1 F.sub.2 F.sub.3 . . . F.sub.L-1 F.sub.L) is detected from the separated signal train. The frame pattern need not be inserted in all MSBs F.sub.1 to F.sub.L of the sub-frames. For example, the frame pattern is inserted in frame bit positions F.sub.1, F.sub.3, . . . , and the remaining frame bits F.sub.2, F.sub.4, F.sub.6, . . . can be used for transmitting information for a transmission line monitor and a service monitor.
In the bit multiplex system shown in FIG. 1, one of the K bits of each frame is used as a frame channel F. In order to obtain a compact, simple circuit, the length of K bits cannot be excessively long. For this reason, an overhead of the frame pattern in the entire transmission data, i.e., a ratio 1/K of the signal component of the frame pattern is a large value. The overhead is expected to increase when the transmission capacity is increased and the bit rate is increased. When system reliability and service of the system are taken into consideration, channels for transmitting information such as a transmission line monitor and a service monitor are required. As a result, the above tendency becomes more conspicuous. In the system (FIG. 2) wherein the frame is divided into sub-frames and the frame pattern is distributed into the sub-frames, a signal train coinciding a specific frame pattern (F.sub.1 F.sub.2 F.sub.3, . . . F.sub.L-1 F.sub.L) must be detected from the separated signal trains to detect synchronization, thereby guaranteeing frame and sub-frame synchronization. Transmission line monitor information and service monitor information are inserted in the frame bits F.sub.1 to F.sub.L, or the number L of sub-frames within one frame or the number I of bits of the sub-frame is increased to allow information transmission having a small overhead with respect to the total volume of data to be transmitted, without complicating the circuit arrangement. However, once a synchronization error occurs, hunting for a maximum of one frame is required to detect a signal train coinciding with the frame pattern (F.sub.1 F.sub.2 F.sub.3, . . . F.sub.L-1 F.sub.L) from the separated signal trains, and a maximum synchronization time required for restoring synchronization is L.times.I.times.1 frame (seconds). Therefore, if the number L of sub-frames or the number I of bits of the sub-frame is increased, the average time required for detecting the frame pattern (F.sub.1 F.sub.2 F.sub.3, . . . F.sub.L-1 F.sub.L) upon occurrence of a synchronization error is undesirably prolonged.